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Author Topic:   Engine Speed at Optimum Cruise
AKwalor posted 06-04-2004 05:38 PM ET (US)   Profile for AKwalor   Send Email to AKwalor  
At what speed should an outboard engine be run to prolong its life and produce optimum fuel economy at cruising speed?
Moe posted 06-04-2004 06:48 PM ET (US)     Profile for Moe  Send Email to Moe     
Keep most of your operating time at or below 70-75% of redline, i.e. 4200-4500 rpm for a 6,000 rpm motor. It's good to regularly run WOT for about a minute every trip or so to identify potential problems in the motor. They'll often show up under the pressure of WOT some time before they will at lower rpms and throttle settings.

--
Moe

Liteamorn posted 06-04-2004 07:37 PM ET (US)     Profile for Liteamorn  Send Email to Liteamorn     
I also run my outboards around 4000-4500 . If you listen to the motor while running you will find the "sweet spot". Thats where they seem to WANT to be run at. She won't be screeming or working hard.
Peter posted 06-05-2004 09:18 AM ET (US)     Profile for Peter  Send Email to Peter     
For as long as I can remember, the general rule of thumb has been to run at two-thirds to three-quarters throttle (I assume that is to be measured as a function of engine speed).

I have found that on the 2-cycle loop charged outboards that I have had that have a WOT maximum RPM of 5500, cruise speed seems to settle between 3700 and 4000 RPM. On the 2-cycle loop-charged motors that I have had that have a WOT maximum RPM of 6000, cruise speed seems to settle between 4000 and 4300 RPM.

Of course, alot of this depends upon what the HP to weight ratio is and the pitch of the propeller. In my cases, the power to weight ratio on the various Whaler hulls I've had has been about the same and the boats have been propped to always be in the upper half of the WOT range at WOT as typically loaded.

One thing I've observed is that variable ratio oil injected 2 cycle outboards seem to use much more oil above 4000 RPM than they do below that threshold.

prm1177 posted 06-06-2004 01:14 PM ET (US)     Profile for prm1177  Send Email to prm1177     
On most boats I've had I have noticed that there is a point after planing where the engine seems to run quieter and easier. I usually cruise at that RPM (around 3200 on my [incomprehensible abbreviation]). It appears that the engine load dramatically decreases at that speed. This gets me a comfortable ride and excelent fuel economy.

Interestingly, there is a second "point of ease" at around 4800 RPM where the same conditions apply. To all you physics majors out there. Any ideas why?

jimh posted 06-07-2004 08:39 AM ET (US)     Profile for jimh  Send Email to jimh     
Using the engine's sound as an indicator of optimum performance is a good technique, although these days with many engines having fuel flow measuring sensors it is possible to get instantaneous readings of optimum fuel economy and to adjust the engine speed to optimum for the current load and conditions.

It seems completely intuitive that engine life would correspond roughly with the total piston travel over time, and any reduction in total piston travel would increase engine life. Therefore the slower an engine was run the longer it would last. Certain speeds, typically in the range of 2000-3000 RPM often result in a non-hydroplane condition and put the engine under a heavy load. Avoiding these speeds as much as possible helps increase engine life. I have been told that it is possible, when running on excessively lean fuel mixtures and "lugging" the engine like this, to cause severe heating in the combustion chamber in a very short time. This situation can lead to engine failure in a few minutes and must be avoided.

The particular engine speed at optimum cruise will depend on many factors, including the engine horsepower, the propeller pitch, the lower unit gear ratio, the boat weight, the sea state, etc., but anecdotal reports of speeds in the range of 3500-4000 RPM are in good agreement with my own observations.

Peter posted 06-07-2004 01:19 PM ET (US)     Profile for Peter  Send Email to Peter     
Interestingly, I just installed a NAVMAN 3100F fuel flow monitor (made by Navman NZ Limited, a Brunwick owned subsidiary) on my 27 Whaler. Maximum fuel economy happens to occur right where I thought the good cruise speed was and where the engines sounded their best to my ears (4000 to 4300 RPM).

elaelap posted 06-09-2004 10:28 AM ET (US)     Profile for elaelap  Send Email to elaelap     
I posted last month about the sluggish response range between about 1900-2900 rpm with my new Outrage 18/Yamaha 115 four stroke combination, where it feels as though the boat is plowing ineffectively through the water. It concerned me greatly at first; now I just swiftly power through that range or stay below it, and my concerns have evaporated. My boat/motor combination planes at 3000-3100 rpm, and hits a sweet spot at about 33-3400 rpm where I can feel the boat suddenly GLIDE through the water on plane, and I sense that fuel economy has greatly increased and wear and tear on the motor has greatly decreased. Hanging just barely on plane--in my case between 3000-3100 rpm--puts the boat on the 'wrong slope of the power curve' (I think that's the correct phrase), and while she's planing, she very obviously doesn't feel as free and smooth as she suddenly does with a tiny increase in turns.

After repowering I quickly found a great cruising range for the boat/motor: 3700-4000 rpm gives me 22+ - 25 mph at sea, and a couple of miles per hour more on a flat, windless lake. I haven't checked fuel consumption yet (in fact I have concerns about filling the internal tank completely to accurately test fuel usage, concerns I'll raise today in a new topic), but so far so good, I think, as long as I stay away from the 'dead zone.'

Tony

BQUICK posted 06-09-2004 10:53 AM ET (US)     Profile for BQUICK  Send Email to BQUICK     
On Mercs the best point was where max ignition advance occurred....interestingly corresponded with the throtte lever being level. Whether this was planned or not ....beats me.
I would pull the engine cover and see what moves as you move the throttle.
Newer motors with computer controlled timing....who knows.

Bruce

Ed Stone posted 06-09-2004 07:07 PM ET (US)     Profile for Ed Stone  Send Email to Ed Stone     
The smartcraft gauges on my 135 Opti's show
the best fuel burn at about 3500 rpm's.
I generaly run at 4000 to 4300 rpm's or
31 to 34 mph.

Ed Stone

flawton posted 06-10-2004 11:48 AM ET (US)     Profile for flawton  Send Email to flawton     
My 90 yamaha 2s (17' newport) has a sweet spot at 4000 rpms. The 4000 rpm cruise speed is the farthest throttle point that the throttle will stay without having to hold it in position which makes it easy to find.
Clark Roberts posted 06-12-2004 06:59 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Flawton, check and see if your throttle has a "friction" adjustment (usually a screwdriver adjustment) and tighten up a bit so that the lever stays where you put it. On the benicle type control access to this adjustment is usually under the snap-off cover. Happy Whalin'... clark .. SCN
Marsh posted 06-12-2004 01:30 PM ET (US)     Profile for Marsh  Send Email to Marsh     
I typically run between 5,800 and 6,000rpms - I need all the speed I can get to try to keep up with all of the pontoons and jon boats on area lakes (forget keeping up with bass boats). I guess I am sacrificing a bit of engine life, as well as fuel consumption, but, I just can't stand everyone making fun of the slow-a$$ whaler.

Later,
Marsh

flawton posted 06-12-2004 07:40 PM ET (US)     Profile for flawton  Send Email to flawton     
Thanks for the info CR, I will look into that. I sorda actually like it the way it is cause it keeps me from running to hard and burning alot more fuel.
Bigshot posted 06-15-2004 11:20 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I try and stay under 4k for economical reasons. Depending on boat/motor combo....follow the sweet spot but see if she has one under 4k. With 1 person my 704s on my montauk is barely audible at 3500, with a load I need to creep it up to 3800 or so.

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